MODELS FOR BACTERIAL PHOTOSYNTHESIS: ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET BACTERIOPHEOPHYTIN TO METHYL VIOLOGEN AND m-DINITROBENZENE

Abstract. As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC1₂) and to m-dinitrobenzene (m-DNB) in solution. Both MVC1₂ and m-DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh ⁺. The quenching of BPh* probably results from electron transfer, which generates short-lived radical pairs involving the BPh radical cation (BPh⁺) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPh^T is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC1₂ or m-DNB. Measurements of the quantum yield of the free BPh⁺ radical indicate that about 3/4 of the BPh⁺ MV⁺] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the BPh⁺m-DNB ⁺] radical pairs must decay by reverse electron transfer, because free BPh⁺ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the BPh⁺ MV⁺] radical pair can be explained by coulombic repulsion. The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 10³-times slower in the radical pair formed in the bacterial reaction center than it is in BPh⁺m-DNB^?], and more than 10⁴-times slower than in BPh⁺ MV⁺]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.